I think the best way to reach that amount is through trading and especially with altcoins, as I think 7 btc is very good amount to invest and you can expect higher profits even with a small fluctuation.
Altcoin trading is a risky way of increasing your coins,if he is not experienced he could get stuck with all the pump and dump coins and will be holding everything,investing in a bitcoin related company is a good way,find out the best sites who are taking outside investment.
Just buy another 10 extra Bitcoin so that you have 17 in total, it is going to be pretty hard to try and earn another 10 Bitcoin for free.
You can also try it with gambling but I do not recommend that.
that is a funny suggestion,if he was to purchase the rest of the coins he would have done that and he doesn't need an advice for that ,gambling is not a way to increase your coins